1. Field of the Invention
This invention relates to a crucible having superior creep resistance and comprised of an iridium-rhenium alloy, and to a process for growing a crystal, such as a gadolinium-gallium-garnet crystal, in such crucible, and to a process for preparing such a crucible.
2. Description of the Prior Art
(a) Defect-Free Gadolinium-Gallium Garnet
Magnetic bubbles are tiny, cylindrical, magnetized areas on computer chips. They are usually contained in several micrometer-thick films of garnet material. Such bubbles can be moved around electronically so that they can be used for computing and for storing memory.
In a typical magnetic bubble domain storage device, there are at least two layers of material: a gadolinium-gallium-garnet substrate layer, and, on top of the substrate layer, a bubble formation layer. See "Bubble Storage Density Increases Fourfold", Industrial Research/Development, May, 1979, pages 45-46.
The bubble domain storage devices must be defect-free. A defect in a magnetic bubble film is anything which will stop or deflect a bubble. Defects include magnetic particles in the film, cracks, scratches, pits, grain boundaries, and dislocations in the crystal structure. A defect-free film must be a nearly perfect, dislocation-free single crystal; and this means that a nearly perfect, dislocation-free substrate is required to support the film.
Single-crystal magnetic garnet films can be deposited only on other garnet single crystals by a process called epitaxy (the growth of one crystal upon another). The substrate garnet crystal must be transparent so that bubbles in the film can be seen for testing purposes. The lattice structure of the substrate must be as near in size as possible to that of the film so that the substrate and film can join to form a coherent interface. One of the few substrates which meets these requirements is gadolinium-gallium-garnet. Levinstein et al., Bell Laboratories Record, July/August 1973, pages 209-214.
Impurities in the gadolinium-gallium-garnet crystal tend to stop or deflect the magnetic bubbles of the bubble domain device and limit the usefulness of the garnet in said device. Consequently, it is important to use a crucible during the growth of the gadolinium-gallium-garnet crystal which is compatible with and does not contribute impurities to the crystal melt.
(b) The Use of Iridium Crucibles to Grow Gadolinium-Gallium-Garnet
Gallium oxide and gadolinium oxide can be charged to an iridium crucible in order to form a melt from which a gadolinium-gallium-garnet crystal can be pulled. The gallium oxide is a reactive component, and it tends to react with the iridium crucible. Thus, for example, when gadolinium-gallium-garnet crystal is grown at a temperature of 1725.degree. C. using a 90 volume percent nitrogen/10 volume percent oxygen ambient gas, iridium losses of from 0.5 to 1.0 grams per hour have been recorded. B. Cockayne, Czochralski Growth of Oxide Single Crystals, Platinum Metals Review, Vol. 18, July, 1974, pages 86-91.
(c) Crucible Creep
U.S. Pat. No. 3,210,167 discloses that crucible creep is a physical action which occurs in a crucible at high temperatures and under the prolonged action of small forces; under these conditions, a progressive deformation of the crucible occurs which eventually is followed by the appearance of cracks. One of the reasons creep occurs is because of the corrosive action of oxygen which is present in and around the crucible when it is used to heat molten materials to high temperatures.
When an iridium crucible is used in an oxidizing atmosphere, creep is caused by the preferential oxidation of iridium at the grain boundaries. Some of the iridium oxide formed may be lost to the crystal melt, and the crucible is weakened by the loss of the material comprising it. Furthermore, even those particles of iridium oxide which remain on the crucible impart less strength to the crucible than do iridium particles.
A crucible consisting of a metal which has a poorer oxidation resistance than iridium should suffer substantially higher metal losses under high temperature oxidizing conditions and, thus, should be more prone to suffer from creep than an iridium crucible.
(d) The Oxidation Resistance of Rhenium
The oxidation resistance of the refractory noble metals, such as rhenium, ruthenium, iridium, rhodium, platinum, and palladium, ranges from among the best that is known (that characterized by rhodium) to the worst known (that of rhenium). The mechanism of oxidation of the noble metals involves the formation of a volatile oxide and metal loss due to oxide vaporization and metal vaporization at temperatures above the oxide decomposition temperature. At temperatures below the oxide decomposition temperature, a stable oxide film is formed. The decomposition temperature (the temperature at which the dissociation pressure of the solid oxide equals 1 atmosphere) of iridium oxide is 1100.degree. C. One of the rhenium oxides, ReO.sub.3, melts at 160.degree. C. and disproportionates to ReO.sub.2 and Re.sub.2 O.sub.7. Re.sub.2 O.sub.7, melts at 296.degree. C. Refractory Metals and Alloys, Met. Soc. Conf. (Interscience, New York, 1961), Vol. 1, pp. 407-409.
A comprehensive study of rhenium alloys is presented in Savitskii et al., Rhenium Alloys, IPST Cat. No. 551 (Israel Program for Scientific Translations, Ltd., Jerusalem, 1970), available from the U.S. Department of Commerce, Clearinghouse for Scientific and Technical Information, Springfield, Va., publication TT 69-55081. Savitskii et al. disclose that rhenium is characterized by ". . . rapid disintegration due to intense oxidation at high temperatures . . . " that, when ". . . heated above 600.degree. C. rhenium reacts vigorously with oxygen to form rhenium heptoxide . . . " (page 343), and that ". . . the oxidation of rhenium at the grain boundaries on heating in air causes intergranular failure during hot working . . . " (page 345).
It is an object of this invention to provide a crucible useful for the preparation of gadolinium-gallium-garnet crystals which has superior compatability and creep resistance properties. It is another object of this invention to provide a process for the preparation of said crystals which produces a substantially defect-free crystal with little or no crucible deformation.